Light emitting diode (LED) lighting systems are becoming more prevalent as replacements for existing lighting systems. LED systems are an example of solid state lighting (SSL) and have advantages over traditional lighting solutions such as incandescent and fluorescent lighting because they use less energy, are more durable, operate longer, can be combined in multi-color arrays that can be controlled to deliver virtually any color light, and generally contain no lead or mercury. A solid state lighting system may take the form of a lighting unit, light fixture, light bulb, or a “lamp.”
An LED lighting system may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs), which may include inorganic LEDs, which may include semiconductor layers forming p-n junctions and/or organic LEDs (OLEDs), which may include organic light emission layers. Light perceived as white or near-white may be generated by a combination of red, green, and blue (“RGB”) LEDs. Output color of such a device may be altered by separately adjusting supply of current to the red, green, and blue LEDs. Another method for generating white or near-white light is by using a lumiphor such as a phosphor. Still another approach for producing white light is to stimulate phosphors or dyes of multiple colors with an LED source. Many other approaches can be taken.
An LED lamp may be made with a form factor that allows it to replace a standard incandescent bulb, or any of various types of fluorescent lamps. LED lamps often include some type of optical element or elements to allow for localized mixing of colors, collimate light, or provide a particular light pattern. Sometimes the optical element also serves as an envelope or enclosure for the electronics and or the LEDs in a lamp. LED lamps and LED light fixtures can used either transmissive optical elements or reflective optical elements. For example, a so-called “troffer” style ceiling fixture includes a reflector that serves and an optical element, and in some circumstances may include additional optical elements such as glass plates or lenses.
Color reproduction can be an important characteristic of any type of artificial lighting, including LED lighting. Color reproduction is typically measured using the color-rendering index (CRI). The CRI is a relative measurement of how the color rendition of an illumination system compares to that of a theoretical blackbody radiator. In practical terms, the CRI is a relative measure of the shift in surface color of an object when lit by a particular source. The CRI equals 100 if the color coordinates of a set of test surfaces being illuminated by the lamp are the same as the coordinates of the same test surfaces being irradiated by the theoretical blackbody radiator. Daylight has the highest CRI (100), with incandescent bulbs being relatively close, and fluorescent lighting being less accurate (70-85). Certain types of specialized lighting, such as mercury vapor and sodium lights exhibit a relatively low CRI (as low as about 40 or even lower).
An LED lamp or fixture may need to be self-contained. In such a case, a power supply is included in the lamp structure or fixture along with the LEDs or LED packages and the optical components. A heatsink is also often needed to cool the LEDs and/or power supply in order to maintain appropriate operating temperature. The power supply and especially the heatsink can often block some of the light coming from the LEDs in at least one direction. If the bulb of fixture is intended as a replacement for a traditional bulb or fixture, this blocking of light can cause the solid-state system to emit light in a pattern that is substantially different than the light pattern produced by the traditional light source. Optical elements for solid state lighting systems are sometimes shaped or placed to compensate for such effects.